Patent Application
20110303135
1. Field of the Invention
This invention relates generally to a steam generating system having a fossil fuel fired boiler and a regenerative air preheater. More particularly, the present invention relates to a steam generating system having a fossil fuel fired boiler and a rotary regenerative air preheater that exhibits reduced fouling during varying boiler operating levels.
2. Discussion of Prior Art
During the combustion process in the boiler, the sulfur in the fuel is oxidized to SO2. After the combustion process, some amount of SO2 is further oxidized to SO3, with typical amounts on the order of 1 to 2% going to SO3. The presence of iron oxide, vanadium and other metals at the proper temperature range produces this oxidation. Selective catalytic reduction (SCR) is also widely known to oxidize a portion of the SO2 in the flue gas to SO3. The catalyst formulation (primarily the amount of vanadium in catalyst) impacts the amount of oxidation, with rates ranging from 0.5% to over 1.5%. Most typical is around 1%. Therefore plants firing a high sulfur coal with a new SCR can see a large increase in the SO3 emissions, which produce a visible plume, local acidic ground level problems and other environmental issues.
Rotary regenerator heat exchangers are commonly used on a large fossil fuel fired boiler to transfer heat from the hot flue gasses to the cooler input air that is provided to a combustion chamber of the boiler. This type of heat exchanger is typically referred to as an air preheater. The purpose of an air preheater is to increase the efficiency of the fossil fueled boiler. Fundamentally, a rotary regenerative air preheater consists of a large cylinder packed with a plurality of spaced sheets of metal. The sheets are separated from one another to allow hot flue gases to flow over the surface of each plate parallel to the axis of the cylinder, heating them. The hot sheets are rotated into the cooler input air stream to heat the input air. The flue gases and input air usually flow through the air preheater in opposite directions. The entire cylinder is continually rotated around its axis so that the hot gas and the cold air flow alternately over the same metal sheets.
The products of combustion of a fossil fuel often contain both sulfur trioxide (SO3) and water vapor (H2O) so that when the exhaust gas is cooled to a sufficient degree within the air preheater, the SO3 combines with water vapor and condenses into liquid sulfuric acid (H2SO4). This occurs when the temperature of surfaces, such as a heat exchange element of an air preheater, is below the dew point of sulfuric acid. When ash particles and sulfuric acid are both deposited on the metal surfaces in the air preheater, they stick to the metal surfaces and cause a phenomenon called fouling. Fouling degrades the efficiency of the air preheater by restricting the amount of air and gas flowing through the air preheater.
High velocity jets of steam or air are periodically directed at the metal surfaces to remove the ash/acid deposits in a process known as sootblowing. Sootblowing removes some, but not all, of the deposit from the metal sheets.
The cold ends of regenerative air preheaters are often below the dew point of the H2SO4 in the flue gas, causing a portion of the H2SO4 to condense on the surfaces of the heat exchange elements. As the condensed ash and H2SO4 accumulate, they create a pressure drop in the flow through the heat exchanger 100. The pressure drop becomes larger over time as solids such as ash or other solid material from the combustion of the fuel also accumulate on the heat exchange elements. If the fouling is severe enough, the flow passages between metal sheets may become plugged. In this event, heat transfer surface area is lost and the fan may be incapable of moving the necessary amount of combustion air through the air preheater.
The cold end of an air preheater, by nature of the lower gas temperature, has a higher gas density and hence a lower flow velocity. Typically the cold end flow velocity is only about 60% of the hot end flow velocity. Lower gas velocities also result in more fouling.
Other factors also add to fouling, such as low boiler load. Low boiler load causes the velocity to drop to a velocity that can be as low as 25% of hot end maximum continuous rating (MCR).
Currently, there is a need for an air preheater that resists fouling under varying combustion conditions.
Briefly stated, the invention in a preferred form is an air preheater that is more resistant to ‘fouling’ under varying boiler loads.
It is an object of the invention to provide an air preheater that is more resistant to corrosion.
It is an object of the invention to provide an air preheater that adjusts to varying boiler loads.
It is an object of the invention to provide an air preheater that adjusts flue gas velocity under varying boiler loads.
Other objects and advantages of the invention will become apparent from the drawings and specification.
The present invention may be better understood and its numerous objects and advantages will become apparent to those skilled in the art by reference to the accompanying drawings in which:
The majority of steam-generating systems utilize stationary or rotary regenerative air preheaters to increase the boiler efficiency. The most common being rotary regenerative air preheaters. This type of air preheater features rotating heat exchange elements. The present invention relates to boiler systems equipped with either type of regenerative air preheater. To facilitate discussion, the inventive arrangement will be discussed in combination with a rotary regenerative air preheater.
With reference to
In a typical rotary regenerative heat exchanger 100, the flue gas stream 224 and the combustion air inlet stream 230 enter the rotor 112 from opposite ends and pass in opposite directions over the heat exchange elements 142 housed within the heat exchange element basket assemblies 122. Consequently, the cold air inlet 130 and the cooled flue gas outlet 126 are at one end of the heat exchanger, referred to as the cold end 144, and the hot flue gas inlet 124 and the heated air outlet 126 are at the opposite end of the air preheater 100, referred to as the hot end 146. Sector plates 136 extend across the housing 114 adjacent the upper and lower faces of the rotor 112. The sector plates 136 divide the air preheater 100 into an air sector 138 and a flue gas sector 140.
The arrows of
As stated above, additional acidic fouling of the cold end 144 of the air preheater 100 creates a larger pressure drop across the air preheater 100. Particulate matter carried in the flue gas also accumulates over time on the surfaces of the heat exchange elements 142, and the presence of these deposits adds to the pressure drop of the air preheater. This particulate matter tends to accumulate predominantly in localized areas of low flow velocity.
Therefore, fouling is due to two problems:
1) condensation of acids that accumulate fly ash and other particulates; and
2) regions of low velocity flow that become lower at low boiler loads.
There have been attempts to overcome each of the problems in different ways. One device functioned to partially block only the flue gas inlet. This had disappointing results. At that time all of the factors leading to fouling were not recognized and addressed.
The present invention addresses both the acid condensation problem and the velocity-related fouling problem. High velocity streams of particles erode solid materials in a process similar to sand blasting. The rate of erosion is proportional to the velocity raised to a power greater than 1. Our experience is that fly ash erosion is proportional to the flow velocity raised to the 3.4 power.
Therefore, it would be beneficial to increase the flow velocity in the gas sector to reduce the amount of deposit on the heat exchange elements 142. Increasing the flow velocity in the air sector does not appreciably aid in removing deposits because there is little to no particulate matter in the air sector. However, reducing the amount of heat transfer surface in the air sector does serve to raise the gas temperature in the gas sector, which results in less acid condensation and therefore less fouling.
The air flow into the boiler is related to the operating level of the boiler. Therefore, a boiler running at 60% of its maximum continuous rating (MCR) would require and take in less combustion air than the same boiler running at 90% of MCR. Consequently, a boiler running at 60% MCR would exhaust less flue gas than one running at 90% MCR. The smaller amount of flue gases exiting through the same cross section with approximately the same density, would exit at a lower velocity.
Also, when the boiler is running at 60% MCR. vs. 90% MCR, it produces flue gases that exit with a lower temperature. Therefore, boiler operation level affects the input air velocity into the boiler, the exhaust flue gas flow velocity out of the boiler and temperature of the exiting flue gases.
Referring now to
By restricting flow into both the flue gas inlet 124 and the air inlet 130 of the air preheater, a smaller effective area for heat transfer will result in less heat exchanged. This causes a greater portion of the metal surfaces to have a temperature above the sulfuric acid dew point, thereby reducing fouling of the metal surfaces. Also, the flow velocity in the gas sector is increased, which facilitates the erosion of any accumulated deposit.
Furthermore, if the cold air flowing into the air preheater is heated by another heat exchanger in order to keep the metal temperatures above the acid dew point, then obstructing flow of both the air and gas side of the air preheater 100, the amount of heat required from the other heat exchanger will be reduced. This will save energy overall since obstructing a portion of the metal surfaces on the air side requires a negligibly small amount of energy compared to the amount that would be needed to heat the cold air to a sufficient degree.
The damper system 50 also includes an air damper assembly 162 positioned inside the preheater cold air inlet 130, as close to the face of the elements in the basket assemblies 122 as possible to minimize air leakage. Air damper assembly 162 can be partially closed at reduced load conditions of boiler 148 to effectively reduce the flow area of the air inlet 130 and thereby decrease the effective surface area for heat transfer to the air flowing into the air preheater 100. This means that there is less cooling of the cold end (144 of
Due to the increased flow velocity in the flue gas sector (140 of
Also, since less surface area is used to extract heat from the flue gases, the flue gases passing through the air preheater to the cold end are hotter and therefore a larger percentage of the plates in the cold end are maintained above the H2SO4 dew point. This results in less condensation of H2SO4 on the heat exchange elements (142 of
Controller 158, preferably a programmable logic controller (“PLC”) with preprogrammed control logic, monitors the load of the boiler 148 and controls the actuation of the damper blades in the damper assemblies 152, 162.
In a preferred embodiment, the controller 158 receives a signal from the plant distributed control system (DCS) 160. The DCS 160 can determine the operating load of the boiler 148, based on the monitored parameters, and can be programmed to send a signal indicating the boiler load to the controller 158. Upon receiving the signal, the controller 158 will calculate the boiler load and actuate dampers assemblies 152, 162 accordingly.
Now with respect to both
Similarly, as the air damper assembly 162 closes more of the air inlet 130, the velocity of the inlet air stream 230 increases. Closing more of the air inlet 130 also reduces the surface area of the heat exchange elements 142 that are exposed to the air inlet stream 230. This results in less heat being absorbed by the air inlet stream 230, again causing flue gas outlet stream 226 to have higher temperatures exiting the air preheater.
The increased velocity of the flue gas passing through the air preheater 100 tends to erode accumulated deposit in the air preheater at a rate based upon the velocity raised to the 3.4 power. The controller may operate the flue gas damper 152 and the air damper 162 to maximize the erosion of accumulation, however, the damper assemblies may not be closed to the degree that allows the exiting flue gas to exceed a maximum allowable temperature. This temperature may be predetermined based upon the maximum temperatures that the downstream equipment can safely tolerate along with a desired margin of safety.
With reference to
As shown in
Flue gas damper assembly 152 has the same parts and operates in the same manner as that described for air damper assembly 162. Therefore, the description above applies equally for flue gas damper assembly 152 as applied to the flue gas inlet instead of the air inlet.
The controller 158 operates the actuator 166 of the damper sections 163, 164 to partially restrict flow in certain areas of the flue gas inlet (124 of
It should be appreciated that a regenerative air preheater 100 in accordance with the invention may include more, or fewer damper assemblies 152, 162 shown in
Referring now to
Referring now to
In addition, a larger pressure drop at 100% power would require a higher capital cost for higher-pressure air and gas fans 188 and higher operating costs for running the larger motors that these larger fans 188 would require. For all but the worst coals, the plant data measurement system does not show an increase in pressure drop at full load over the 8 hours of time between soot blowing cycles. The fouling that is observed is either hot end fouling from large particles of “popcorn” or slag that has formed on some hotter upstream surface, dislodged and is carried by the flue gas stream, or cold end fouling which may be acidic fouling and/or particulate fouling in low velocity and low turbulence zones.
However, at low load conditions the gas outlet temperature is always lower than the gas outlet temperature for the MCR design point. This is due to two factors. At lower boiler load conditions, the temperature of the flue gas entering the air preheater 100 is lower than at the design point. The air preheater 100 is also more efficient, since the flue gas velocity is also lower and the resultant decrease in heat transfer coefficient has a lesser effect than the reduction in flow for the existing surface area, therefore producing a greater reduction in flue gas temperature. Often the lower temperature that occurs at low loads is sufficiently low to result in the condensation of sulfuric acid. Some plants use steam air heating to raise the inlet air temperature, and therefore the exit gas temperature and element plate temperatures to avoid condensing acid. However, the accumulation of dust due to the reduced velocity is not mitigated by this procedure.
Table 1 compares flue gas velocity of the present invention against a conventional air preheater during a thirty percent (30%) load condition, a seventy percent (70%) load condition and MRC. Damper assemblies 152, 162 according to the present invention are used to effect a fifty percent (50%) reduction in the flue gas inlet flow area to produce significant increases in the velocity of the flue gas flow. As can be seen, doubling the inlet velocity of the flue gas doubles the outlet velocity of the flue gas, with a proportional increase in the average flue gas outlet velocity to the 3.4 power. A ratio of the average flue gas outlet velocities to the 3.4 power at 70% power and MCR of 3.54 is achieved with the subject invention compared to a ratio of 0.32 for a conventional air preheater. For a 30% power level the ratio for the subject invention is 0.43 compared to 0.04 for a conventional air preheater. Closure of additional dampers 56 would provide even higher cold end velocities and cleaning effect (velocity to 3.4 power) at the 30% load case. The conditions of this example are not necessarily optimal conditions but merely illustrate the principle of the invention.
In alternative embodiments, the dampers may be actuated by gear drives, belt drive, chain mechanisms, solenoids or other known actuator mechanisms. These all fall under the scope of the present invention.
While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation.
